The invention relates to a nuclear reactor system which includes means for injecting additional coolant into the reactor coolant circuit, and in particular to a system wherein the residual heat removal apparatus which thermally couples the coolant circuit system to heat exchangers for removing residual heat is coupleable into fluid communication between an in-containment refueling water supply tank and the reactor coolant circuit as a means to inject additional coolant. The invention is applicable to reactor systems having passive safety features, with depressurization of the reactor coolant circuit to facilitate injection of additional coolant water.
A nuclear reactor such as a pressurized water reactor circulates coolant at high pressure through a coolant circuit traversing a reactor vessel containing nuclear fuel for heating the coolant, a steam generator operable to extract energy from the coolant. A residual heat removal system is typically provided to remove decay heat during shutdowns. In the event of a loss of coolant, means are provided for adding additional coolant. A coolant loss may involve only a small quantity, whereby additional coolant can be injected from a relatively small high pressure makeup water supply, without depressurizing the reactor coolant circuit. If a major loss of coolant occurs, it is necessary to add coolant from a low pressure supply containing a large quantity of water. Whereas it is difficult using pumps to overcome the substantial pressure of the reactor coolant circuit (e.g., 2,250 psi or 150 bar), the reactor coolant circuit is depressurized so that coolant water can be added from an in-containment refueling water storage tank at ambient pressure in the containment shell.
The Westinghouse AP600 reactor system, of which the present invention is a part, uses a staged pressure reduction apparatus for depressurizing the coolant circuit. A series of valves couple the reactor outlet (also known as the "hot leg" of the coolant circuit) to the inside of the containment shell. The valves operate at successively lower pressures. When initially commencing depressurization, the coolant circuit and the containment structure are coupled by depressurization valves through one or more smaller conduits along a flow path with not-insubstantial back pressure. As the pressure in the coolant circuit drops, additional conduits are opened by further depressurization valves in stages, each stage opening a larger and/or more direct flow path between the coolant circuit and the containment shell.
The initial stages couple a pressurizer tank which is connected by a conduit to the coolant circuit hot leg, to spargers in an in-containment refueling water supply tank. The spargers comprise conduits leading to small jet orifices submerged in the tank, thus providing back pressure and allowing water to condense from steam emitted by the spargers into the tank. The successive depressurization stages have progressively larger conduit inner diameters. A final stage has a large conduit that couples the hot leg directly into the containment shell, for example, at a loop compartment through which the hot leg of the reactor circuit passes. This arrangement reduces the pressure in the coolant circuit expeditiously, substantially to atmospheric pressure, without sudden hydraulic loading of the respective reactor conduits. When the pressure is sufficiently low, water is added to the coolant circuit by gravity flow from the in-containment refueling water supply tank.
Automatic depressurization in the AP600 reactor is a passive safeguard which ensures that the reactor core is cooled even in the case of a major loss of coolant accident such as a large breach in the reactor coolant circuit. Inasmuch as the in-containment refueling water storage tank drains by gravity, no pumps are required. Draining the water into the bottom of the containment building where the reactor vessel is located, develops a fluid pressure head of water in the containment sufficient to force water into the depressurized coolant circuit without relying on active elements such as pumps. Once the coolant circuit is at atmospheric pressure and the containment is flooded, water continues to be forced into the reactor vessel, where boiling of the water cools the nuclear fuel. Water in steam escaping from the reactor coolant circuit is condensed on the inside walls of the containment shell, and drained back to be injected again into the reactor coolant circuit.
The foregoing arrangement has been shown to be effective in the scenario of a severe loss of coolant accident. However, there is a potential that if the automatic depressurization system is activated in less dire circumstances, the containment may be flooded needlessly. Depressurization followed by flooding of the reactor containment requires shutdown of the reactor and a significant cleanup effort.
There is a need for a system which is sufficiently responsive to react appropriately to a major accident, but which also minimizes damage and expense if the situation can be remedied appropriately by addition of coolant in excess of the high pressure makeup supply, or perhaps by an orderly shutdown procedure for effecting repairs. This system must be arranged to complement the passive safety system, without retarding or otherwise adversely affecting the ability of the passive safety system to respond to a real accident.